The internal combustion engines have been designed to work with increasingly higher speeds and loads, thereby requiring sliding bearings, which are able to support these increasingly severer operational conditions, through improved fatigue resistance and anti-sticking characteristics, and which can operate with forged steel or cast iron crankshafts, without the need to protect the bimetallic strip with a lead/tin electrodeposited overlayer.
It is known that the high fatigue resistance characteristics of these aluminum base alloys result from a finer and more uniform distribution of the silicon particles in the aluminum phase.
Nevertheless, the known alloys of these type, such as that described in U.S. Pat. No. 4,696,867, still present a silicon particle distribution which, in spite of a possible addition of small amounts of strontium or sodium to said alloys, does not reach a desirable refining to obtain a load capacity compatible with the operational requirements of the modern internal combustion engines.
The grain size of the known alloys of this type is from about 20 to 50 .mu.m, avoiding the achievement of substantial increases of fatigue resistance in the produced bimetallic strips (see FIG. 3 in the drawings). Besides de inconveniences cited above, it should be observed that the tin percentages of about 8% or more in the alloy composition do not allow said alloy, after its usual casting, to be hot rolled to the desired thickness for the sliding strip, because hot rolling these alloys presents the risk of the tin being squeezed out.
Another deficient aspect of the known solutions refers to the lack of treatment steps of the bimetallic strip, so as to obtain, in the sliding strip, a compound that is capable of hardening the aluminum alloy, increasing even more its fatigue resistance through a more accentuated silicon refining.
These prior art aluminum alloys are cast into billets with a shape which is very different from that to be attained by the sliding strip, thus making difficult and costly the manufacturing process of the bimetallic strip and the achievement of the minimum silicon refining in the alloy structure.